1. Field of the Invention
The present invention generally relates to image sensors. More particularly, the present invention relates to image sensors configured to reduce flicker noise and to methods of manufacturing images sensors to reduce flicker noise.
2. Description of the Related Art
Certain types of image sensors utilize photo detectors to capture incident light and convert the light to an electric charge capable of image processing. Examples include Complimentary Metal Oxide Semiconductor (CMOS) image sensors (CIS). CIS devices are generally characterized by analog sensing circuits coupled to CMOS control circuits. The analog sensing circuits include an array of photo detectors having access devices (e.g., transistors) for connection to word lines and bit lines. The CMOS control circuits may include a timing generator and a variety of image processing circuits, such as row decoders, column decoders, column amplifiers, output amplifiers, and so on. Generally speaking, the configuration of the CIS device is analogous to that of a CMOS memory device.
In the meantime, the gate dielectric layers of MOS transistor devices contained in the control circuits of CIS devices have become thinner with decreased design rules and higher operational speeds. As such, the thin gate dielectric layers are thermally nitrided to embed nitrogen atoms which prevent or inhibit boron penetration from the gate electrodes of p-type MOSFETs into the substrate underlying the dielectric layers. Boron penetration is to be avoided since it can induce shifts and fluctuations in the threshold voltages of the devices and degrade the drive current of the CMOS circuits. The thermally nitrided gate oxide layers are generally formed by injecting oxygen and nitrogen gas into a high-temperature annealing furnace which contains one or more wafers being fabricated into CIS devices.
The present inventors have discovered, however, the nitrided gate oxide layers of the conventional CMOS image sensor are a source of increased flicker noise in CIS devices.